Fiber-reinforced ceramic matrix composite (FRCMC) materials are currently of interest for the fabrication of parts that operate under severe use conditions. FRCMC materials offer improved high-temperature strength, chemical resistance, and wear resistance relative to metallic and polymer materials. Moreover, FRCMC materials generally have higher strength-to-weight ratios than metals. Because of these and other advantageous properties of FRCMC materials, they are increasingly being used or studied for use in advanced pumps, turbomachinery, automotive engines, tooling, and other applications.
While FRCMC materials have many desirable properties, they also are subject to undergoing permanent residual strain when initially placed under load. While not wishing to be bound by theory, it is believed that the reinforcing fibers of the material, which are typically in the form of woven layers of fabric laid atop one another, change orientation slightly when the material is initially placed under load. Accordingly, when the part is initially loaded, microcracking of the ceramic matrix material occurs, allowing the fibers to change their alignment. As the fibers become differently oriented, the part can lengthen in the load direction, much as a woven basket can be lengthened by stretching it. When the initial load is removed, the reoriented fibers do not return to their original positions, but rather tend to remain in their new positions. As a result, the dimension of the part in the load direction permanently increases, and this increase in dimension is termed a residual strain herein.
Thus, a part made of FRCMC manufactured to desired shape and dimensions tends to permanently deform and therefore change shape and/or dimensions over the first several cycles of loading and unloading. The resulting part configuration may be significantly different from the designed configuration. In applications where the part dimensions are critical, this tendency to undergo permanent residual strain can lead to undesirable and even catastrophic consequences. For example, a turbine rotor made of FRCMC may undergo permanent radial expansion during initial spin-up, and thereby alter critical blade tip clearances, which can lead to losses in efficiency, disk imbalance, and/or blade damage.
Accordingly, it would be desirable to provide a method for fabricating a part from FRCMC so as to assure that the part has the desired shape and dimensions when placed into service and that the part will not significantly change shape or dimensions during use.